Entropy Sudoku (PuzzleMadness)

About Entropy Sudoku (PuzzleMadness)

All the normal sudoku rules apply: each row, column and 3x3 box holds 1 to 9 once each. On top of that, the digits are sorted into three groups, low 1, 2 and 3, medium 4, 5 and 6, and high 7, 8 and 9. Any 2x2 square of neighbouring cells you can pick out in the grid must contain at least one low, one medium and one high digit. Since a 2x2 square has four cells, one group always appears twice.

There are no lines or symbols on the grid, so the rule is easy to forget and hard to spot until you use it: two low digits side by side force the two cells next to them to be one medium and one high. The site highlights the four cells of any 2x2 square that breaks the rule. Today's Medium grid, for example, starts with only its given digits and nothing else drawn on it.

The grid works like every sudoku on PuzzleMadness: number buttons above and below it, an eraser, and a pencil mode for candidate digits. On a keyboard, the number keys enter digits, the arrow keys move between cells, Ctrl, Shift or Alt with a number adds a pencil mark, and Delete or Backspace clears a cell.

PuzzleMadness is a free puzzle website run by Dewi, a software developer who started out making games for the PlayStation 2, Nintendo Wii and DS and now builds the site as a hobby. It carries dozens of puzzle types, from classic sudoku to newer logic grids, and each one gets its own dated puzzle every day at each difficulty. Entropy Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's Entropy Sudoku puzzle is shared by everyone who opens it.

The site checks your grid as you go and tells you when it is finished; it does not publish solutions. Each completed puzzle earns a score, and scores feed a monthly and an all-time Entropy Sudoku leaderboard, where only your best attempt at a puzzle counts. Older days stay in the archive, so a missed puzzle can still be played later and still counts toward the current month. Playing needs no account; a free registration gives you a username on the leaderboards and a cloud save. The pages carry adverts.

How to play Entropy Sudoku (PuzzleMadness)

1. Open today's Entropy Sudoku page on PuzzleMadness and choose a difficulty from the menu above the grid.
2. Click a cell and enter a digit with the number buttons above and below the grid or with the keyboard; the eraser button clears a cell.
3. Fill every row, column and 3x3 box with the digits 1 to 9, each once.
4. Make sure every 2x2 square of cells contains at least one low digit (1 to 3), one medium (4 to 6) and one high (7 to 9).
5. Turn on pencil mode (the pencil button, or Ctrl, Shift or Alt with a number key) to jot candidate digits in a cell.
6. The site tells you when the grid is complete and adds your score; registered players appear on the leaderboards.

Screenshots

Tips

• Think in groups before digits: pencil L, M or H in cells first, then narrow down to the exact number.
• Two digits from the same group next to each other mean the other two cells of each 2x2 square around them need the two missing groups.
• Every cell sits in up to four 2x2 squares, so a single placed digit affects a whole ring of neighbours.
• Missed a day? Earlier puzzles are in the archive, and finishing one still counts toward this month's leaderboard.

Questions